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One zone on boiler overheats and blows pressure relief valve

I have a Dunkirk boiler with two zones heating a two story house. The second floor zone has been functioning all along. I had a bad Grundfos pump on the 1st floor. It moved freely with a screwdriver but would not move when calling for heat. I replaced the pump. The new pump comes on and off just fine. While trying to refill and bleed the system I noticed the really old pressure reducing-Auto fill valve was so plugged with debris that it was not working so I replaced that along with installing a new ball valve before it. If I fire up the boiler with only the second story zone running it works fine. Pressure on the boiler is 15 when cold. The cold pressure on the one year old expansion tank reads 15psi cold as well. When I run the second floor zone it runs perfectly. It shuts of at 180 and runs at about 20psi on the boiler. This zone has been functioning fine all week. When I turn on the first floor zone the pump turns on but I get no circulation in the zone unless I manually open the flow control valve. Once open I can hear the water flowing through. Within minutes of doing this the temp runs away and blows the pressure relief valve. I have tried this with the water fill supply both off and on. I am stumped. Any ideas out there?

when it refreshed after my post i see that you have made the connection .. sometimes we flip the cartridge so someone looking at things sees it looking like its going the right way. normally because it is. which reminds me i need to flip the wiring around on a circ before someone tells me its not right. what that is requires an allen key . what is happening for you is you need a couple Mexican Metric crescent wrenches..

You need to check the tank pressure when there is no pressure in the system otherwise it will always read the same as the system pressure . If you have system pressure at zero and the tanks at 15 then the tanks right. If the pressure still goes up when it gets hot you need a bigger tank.